How to Unit Test Custom Command Line Flag Validation in Go?

Validating Command Line Flags in Go Unit Tests

Consider the following code that uses command line flags to configure the format type:

<code class="go">// ... elided for brevity ...</code>

To ensure that the -format flag is set to an expected value, a unit test can be written. The flag.Var function can be employed to customize the processing of flags, allowing for validation and more control over the flag values.

Custom flag handling is achieved by implementing the Value interface:

<code class="go">type formatType string

func (f *formatType) String() string {
    return fmt.Sprint(*f)
}

func (f *formatType) Set(value string) error {
    // Validation logic here
}</code>

Applying this to the format flag:

<code class="go">var typeFlag formatType

func init() {
    // ... elided for brevity ...
    flag.Var(&typeFlag, "format", "Format type")
    flag.Var(&typeFlag, "f", "Format type (shorthand)")
}</code>

To unit test the custom flag validation, consider the following approach found in flag_test.go:

<code class="go">func TestCustomFlag(t *testing.T) {
    // Setup test environment
    origArgs := os.Args

    // Define custom flag type
    type myFlag int
    flag.Var((*myFlag)(nil), "customflag", "Custom flag")

    tests := []struct {
        origArgs  []string
        expValue  int
        expOutput string
    }{
        // ... test cases ...
    }

    for _, test := range tests {
        os.Args = test.origArgs
        // Parse flags
        flag.Parse()
        // Check flag value
        if flagValue := flag.Lookup("customflag").Value.(myFlag); flagValue != test.expValue {
            t.Errorf("Expected %v, got %v", test.expValue, flagValue)
        }
        // Restore args
        os.Args = origArgs
    }
}</code>

In summary, the flag.Var function allows for customization and validation of flag values, which can be unit tested following established patterns.

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